1. Field of the Invention
The present invention relates to coating compositions adapted for application to film substrate materials such as plastic films, for example polyolefin films. Such films include polyethylene, polypropylene, polybutene and the like including mixtures thereof, whereby certain properties of such films are improved. Specifically, uncoated, oriented films such as polypropylene have very high heat-sealing temperatures and an extremely narrow heat sealing range, and especially in the case of uncoated, oriented polypropylene, they exhibit a tendency to disorient and shrink when the prequisite sealing temperatures are applied to the surface thereof. Further, when coating materials are applied to the film to remedy the above deficiencies, although such coatings improve the heat seal characteristics of the coated film, it has been found that when packages are wrapped and sealed with such film, and exposed to a high humidity environment, the strength of the sealed area is greatly reduced or may fail completely. The coating compositions of the present invention, when applied to a substrate film such as oriented polypropylene, for example, not only lower the heat-seal temperatures of such film and increase the heat seal range of such coated films, but also greatly improve the high humidity resistance of such films after they have been sealed together. Additionally, the present coating compositions impart increased stiffness or "handle" properties of the film, good hotslip characteristics, and facilitate the formation of peelable heat seals which permits opening of a package overwrapped with the coated film material of this invention without tearing or destroying the wrapper.
2. Description of the Prior Art
Attempts have been made to produce coatings for thermoplastic film substrates employing various hydrocarbon resins and mixtures of resins and the like to provide, for example, improved heat seal properties, but in many instances such polymeric coatings have had deleterious effects on the coated film product. Such defects include poor blocking characteristics, poor optical properties, poor heat sealability and aging characteristics. Further, certain resin coating compositions of the prior art exhibit poor resistance to exposure or storage at high relative humidity which results in seal deterioration or complete failure of the sealed film under even moderate stresses.